Issue with Collision Detection
According to your own comment, the objects are not colliding at the same time (same frame).

Sensor triggering

All Sensor types will trigger the controller when:


The evaluation state changes from not positive to positive.
The evaluation state changes from positive to not positive.
[True Level Triggering] The evaluation state is positive after [Skip] frames.
[False Level Triggering] The evaluation state is not positive after [Skip] frames.
[Level] The state of the build-in state machine changes.


([Tab] and [Invert] manipulate the evaluation result. The trigger rules apply as above after that manipulation)

Applied to your observation:

f+0:

no object is detected
-> sensor remains not positive no controller gets triggered

f+1:

the first object gets detected
-> the sensor changes status from not positive to positive

according to the above rules -> the controller gets triggered
The controller prints  mydetector.000 detected       [Obj1] to the console

f+2:

the first object still gets detected
-> the sensor does not change state as it is already positive
no rule applies -> no controller gets triggered

f+1+n:

The first and the second object get detected
-> the sensor does not change state as it is already positive
no rule applies -> no controller gets triggered

Sensors detecting multiple objects

With sensors that sense more than one object (collision, near, radar) you often want to know when the detected object set changes. Therefore they have an additional parameter [Pulse].

So you can consider following rule that applies on that kind of sensors:


[Pulse] the detected set of objects changes (e.g. other objects are detected/already detected objects are not detected anymore)


f+1+n:

The first and the second object get detected
-> the set of detected objects changes (=new object detected)
according to the sensor rule -> the controller gets triggered

The controller prints  mydetector.000 detected       [Obj1, Obj2] to the console

Solution

Switch on [Pulse]

(switch off [True Level Triggering])